Would you think it's possible to increase horsepower, torque, and fuel efficiency by about 10 percent with just a tune-up? Neither would we, but that's just what a trip to The Dyno Shop got us.
We wanted to see what kind of gains could be had by dyno-tuning a relatively stock, high-mileage engine typical of what the average four-wheeler drives. Our test mule was an '85 Ramcharger with an Edelbrock Performer manifold, a Carter AFB carb, an MSD coil, a Mopar Orange E.C.U., high-performance plug wires, a 180-degree thermostat, 4.10 gears, and a Flowmaster muffler. The remainder of this 195,000-mile 318 was completely stock.
The first step in dyno-tuning a vehicle is to find out how it's used, what sort of gas it's fed, and the mileage and condition of the engine. Engines in better condition that run higher octane fuel can handle more timing than engines in poor condition that run lower octane fuel. In our case, our Ramcharger sees duty ranging from highway driving, to towing, to slow-speed off-road use. We use 89-octane fuel and we knew that the valve seals were wasted, allowing oil into the cylinders, thereby increasing detonation-causing carbon deposits.
With our vehicle information in hand, Dyno Shop co-owner Mark MacNeil and technician Alex Huerta went to work. They hooked our Dodge up to their Dyno Dynamics 850 dyno, connected the sensors for rpm, vacuum, ignition pattern, and air/fuel ratio, and performed a constant-speed increasing load run. With the tires spinning at 55 mph road speed, increasing amounts of load are placed on the dyno rollers, thus requiring more throttle input to maintain speed. The resulting readout allows the technicians to check the air/fuel ratio throughout all circuits of the carburetor's operating range in simulated real-world driving conditions.
The next step was a full-throttle run to see how much power we were generating. Our little 318 made 143 hp at 4,000 rpm and 204 lb-ft of torque between 2,800 and 3,200 rpm.
MacNeil and Huerta first performed a cranking rhythm test, rather than a compression test, to check the condition of the engine. With a compression test, unless the vehicle is started after checking every other cylinder to retain oil pressure, the lifters can bleed down, causing inaccurate readings. The cranking rhythm test is quick enough that this doesn't happen. With the distributor cap removed, MacNeil cranked the engine over while Huerta watched the rotor. Any increase in cranking speed would indicate less resistance in the cylinder and, therefore, a possible problem. In our case an increase in cranking speed around cylinder No. 5 and No. 7 was noticed. Out came the compression gauge to confirm their suspicions. Cylinder No. 7 checked out at 170 psi, but No. 5 read 145 psi-weak compared to the rest of the engine, but not terminal.
Next, Huerta threaded in a new set of Autolite Platinum AP65 spark plugs. The Autolite Platinums come with a three-year unlimited mileage guarantee. They should provide better fuel economy for a longer period of time because the platinum firing tip reduces gap erosion, ensuring that they will keep the 0.040-inch gap way longer than the plugs that they replaced. He then pulled and cleaned the distributor and carburetor so they could be properly tuned in accordance with the air/fuel and ignition pattern readings provided by the dyno runs.
In an effort to cure a detonation problem, we had tuned our carb to run way too rich. Ideally, the air/fuel ratio should be in the 15:1 range during cruising for optimum fuel efficiency, but this may cause surging on a high-mileage, high-compression engine such as ours. Therefore, Huerta chose to shoot for a slightly richer 14.5:1 ratio. Using the air/fuel ratio printout as a guide, the 0.0705/0.0475-inch primary rods were left alone, but the primary jets were changed from 0.101s to 0.98s and the secondary jets were changed from 0.095s to 0.092s.
While the carb was on the bench, MacNeil recurved the distributor. What is recurving? To put it simply, it's allowing either the vacuum or centrifugal advance to come in sooner or later, thus changing the moment when the spark plug fires. Why? At slow operating speeds, such as at idle, the spark plug can fire later in the compression cycle because the piston is moving more slowly. However, at higher engine speeds the spark plug must fire earlier to ensure complete combustion. While it may only take 15 degrees of crankshaft rotation at idle to complete the combustion process, it may take 35 or 40 degrees of crankshaft rotation at higher engine speeds. The baseball analogy is that you must start swinging sooner to hit a ball traveling 100 mph than you do for one traveling 30 mph. Therefore, the spark plug must fire earlier in the compression cycle at a higher rpm. To accomplish this, the position of the rotor is advanced on the distributor shaft by vacuum and mechanical means to advance the sparks. The advance curve, as set by the factory, is a compromise that is intended to work in a variety of situations. Therefore, for optimum performance it's best to have your distributor tailored for your particular application.
The recurved distributor and re-jetted carb were bolted back in place, the fuel filter was replaced, and the initial timing was changed from 13 degrees btdc to 12. Since the timing would now advance sooner, less initial timing was needed for upper rpm performance. The truck was hooked back to the dyno for the final flogging. As the charts show, the results are quite impressive. Our average horsepower increased 11 percent from 122 to 136 and we picked up at least 12 hp everywhere. Our average torque also increased 11 percent from 194 to 216 with the peak dropping 500 rpm. What's more, we saw an increase of 32-lb-ft at the new torque peak of 2,300 rpm!
So how does it work? It's like a completely different vehicle. The truck now fires right up and idles dead-smooth. Acceleration is very strong across the entire rpm range where before there was hesitation below 2,500 rpm. Accelerating up hill seems effortless, and freeway passing power is far superior than before. The difference in the amount of throttle needed to hold the truck at 70 mph on the freeway is so drastic that it almost feels unnatural to have the right foot that far from the floor. However, the best part is that repeated mileage checks on a 250-mile circuit comprising both highway and city driving went from an average of 12.5 mpg to 14.6 mpg! That's almost a 17 percent improvement in fuel efficiency. The only downside is that we had a slight detonation problem due to the carbon buildup, which required a switch to 92-octane fuel. This was to be expected given this engine's mileage and approximate 9:1 compression ratio, but the extra fuel economy offsets the price of switching from 89- to 92-octane fuel. In fact, even with the added price of buying premium, we're still saving money, but we've got extra power as well!
Timing advance is generally controlled by vacuum and centrifugal means. A vacuum advance unit is connected to a source of engine vacuum. As vacuum increases, an arm advances the timing by rotating the entire internal distributor assembly with respect to the cam. (A) MacNeil set our vacuum advance to 24 degrees. In low vacuum situations, such as wide-open throttle, there is no vacuum advance.
Mechanical, or centrifugal, advance is controlled by weights and springs. As the distributor shaft spins, centrifugal force tries to pull the weights outward. Springs of varying strength hold the weights and keep them from moving at slower rpm's. As speed increases, the centrifugal force overcomes the springs, allowing the weights to move and change the position of the rotor with respect to the cam. (B) Centrifugal advance can be tailored by adding different weights, by swapping lighter or heavier springs, or both. Our lighter primary spring looked good, but our heavier secondary spring was too strong. It kept the advance from coming in soon enough. (C) MacNeil swapped secondary springs until he found one that allowed for 20 degrees of mechanical advance at 4,000 rpm.
If you would like to have your distributor recurved and can provide your engine's compression, cam specs, engine size, and how your vehicle is used you can mail your distributor to The Dyno Shop. They will recurve it using your information and ship it back to you ready to be installed.
Dynos Don't Lie
As you can see by the following graph and chart, we saw increases in all areas. Check out the impressive increase in lower rpm torque!